This invention relates to an improved headering means for a heat exchanger comprising a stacked array of thin-walled heat exchange channel elements.
In the field of heat exchange applications requiring pressure-bearing walls as the primary heat exchanger surface, considerable effort has been expended to develop light weight, inexpensive heat exchange elements. In recent years a number of compact heat exchanger designs have been developed which utilize comparatively thin-walled heat transfer channel elements, e.g., 8-12 mils in thickness, of light weight materials such as aluminum. Such types of heat exchangers have particular utility in automobile radiator and heater applications, where size and weight are primary considerations.
An illustrative heat exchanger construction for the foregoing applications is disclosed in U.S. Pat. No. 3,757,856 issued Sept. 11, 1973 to L. C. Kun, wherein each channel element of the heat exchanger is provided with an isostress contoured heat exchange surface comprising a multiplicity of uniformly disposed outwardly extending projections formed from a portion of each wall surface. These projections have load-bearing segments at their extremities whereby the facing walls of adjacent channel elements are mated in supportive relative with each other. Upon being subjected to a differential pressure across the channel wall, a substantially uniform fiber stress distribution is obtained in the isostress contoured surface. This uniform stress distribution substantially eliminates stress concentration points in the walls of the channel elements thereby permitting the walls to be fabricated from very thin sheets of thermally conductive material.
In such heat exchangers constructed from thin-walled channel elements, wherein the channel elements are stacked in an array to form the heat exchanger core, the provision of low-cost, easily fabricated header means which maintain an efficient fluid-tight seal with the channel elements in the stacked array encompasses specific problems not encountered in headering arrangements in heavier walled systems. With channel elements having pressure withholding walls of lower thickness, there is a lower resistance to heat transfer associated with the walls, in other words, a higher rate of heat transfer per unit weight of wall material, which permits the thin-walled channel elements to be closely spaced together to form a highly compact stacked array. Associated with this degree of compactness are correspondingly small dimensions for the channel elements.
As an example of the above-described structural characteristics of thin-walled channel element heat exchangers, in a heat exchanger constructed with channel elements of the type as disclosed in the aforementioned Kun U.S. Pat. No. 3,757,856 and suitable for use as an automobile radiator, the stacked array may be formed of 150 channel elements each 30 inches long with a cross-section characterized by a 1 inch major axis, a minor axis of 0.12 inch and a wall thickness of 0.008 inch. In such array, the spacing between facing walls of adjacent channel elements may be on the order of 0.120 inch. Thus, the provision of inlet header means joined in flow communication with the channel elements at one end of the array and outlet header means joined in flow communication with the channel elements at the opposite end of the array requires the fluid-tight sealing of numerous header-array joints of exceedingly small dimensions. In addition, the thinness of the channel element walls render them easily susceptible to bending and deformation in the heat exchanger fabrication process.
As a consequence of the foregoing characteristics of thin wall channel element heat exchangers, it is both difficult and expensive to employ conventional headering arrangements such as are used in the fabrication of large-scale heat exchangers. For example, in the construction of commercial tube-and-shell heat exchangers and autombile radiators, it is common practice to employ a tube sheet headering arrangement. In such systems, the tubes in the heat exchanger core assembly are characteristically forced through correspondingly sized openings in a sheet member and the latter is then joined to suitable tank or shell means to form a header chamber communicating with the tubes of the core assembly for introduction or withdrawal of fluid being passed through the tube members. Alternatively, the tube members may be smaller in size than the openings in the tube sheet and after being passed through the openings the tubes are expanded as by swaging or other means to form a fluid-tight seal between the tubes and surrounding sheet. These approaches are not practical in application to thin wall channel element heat exchangers, due to their aforementioned susceptibility to bending and deformation during the associated fabrication steps and the need for extremely narrow dimensional tolerances for both the channel elements and the closely spaced tube sheet openings.
As a result of the inapplicability of conventional large-scale heat exchanger headering designs, a variety of header configurations have been proposed to accommodate the specific structural features of thin wall channel element systems. In the aforementioned Kun U.S. Pat. No. 3,757,856, a tank header arrangement is disclosed wherein comb-shaped members are inserted into the end sections of the channel element stacked array from opposite sides such that the corresponding teeth of the respective combs sealingly overlap one another and serve as spacers between adjacent channel elements. A tank is then suitably attached to the periphery of the comb members at each end of the array to form the respective fluid introduction and exit means. This design, while overcoming the inherent deficiencies of the conventional tube sheet headering arrangement, is nonetheless associated with numerous closely spaced comb member-channel element joints which must be leak-tightly sealed so that in the operational mode a fluid fed through the channel elements will not leak into the space between adjacent elements. Accordingly, each of these individual joints must be bonded, as by adhesive, to insure positive sealing, a step which is tedious, time-consuming and costly.
Another type of headering arrangement which has been proposed for thin-walled channel element stacked array heat exchangers incorporates channel elements having closed ends and flat side walls at the end sections with openings in the side walls for ingress and egress of the fluid being flowed through the channel element. In one such arrangement the header means include manifold tubes passing through the openings in the channel elements, the manifold tubes having flow openings whereby fluid communication is established between the tubes and the channel elements. This arrangement requires fluid-tight sealing of the numerous small joints between the tube and the associated flat side wall portions of the stacked channel elements, which is difficult to achieve economically. Another variant configuration under this arrangement involves bonding of the flat side wall portions surrounding the wall openings on adjacent channel elements to each other in wall to wall contacting relationship. This design is somewhat more advantageous in that the joint surfaces have a relatively large area for bonding as compared to the aforedescribed systems so that it is easier to fabricate; nonetheless, a multiplicity of bonding joints, associated with an exceedingly large aggregate joint length, are again employed each of which must be positively sealed to insure operability of the heat exchanger assembly.
Accordingly, it is an object of the present invention to provide an improved headering arrangement for heat exchangers of the type employing a stacked array of thin-walled heat exchange channel elements.
It is a further object of the invention to provide a heat exchanger assembly of the above type which is easily fabricated and incorporates joints having a relatively low aggregate joint length which must be leak-tightly sealed.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and appended claims.